Supersonic wind tunnel and method of operation



Oct. 29, 1968 J. 1.. HARP, JR

SUPERSONIC WIND TUNNEL AND METHOD OF OPERATION 2 Sheets-Sheet 1 Filed Jan. 5, 1966 INVENTOR fis 4. Hnzp 5 MOLE FRACTION IN PRODUCT GAS Oct. 29, 1968 J. L. HARP, JR 3,407,653

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United States Patent 3,407,653 SUPERSONIC WIND TUNNEL AND METHOD OF OPERATION James L. Harp, Jr., Canoga Park, Calif., assignor to The Marquardt Corporation, Van Nuys, Calif a corporation of California Filed Jan. 5, 1966, Ser. No. 518,812 Claims. (Cl. 73-147) ABSTRACT OF THE DISCLOSURE A supersonic wind tunnel and operating method therefor wherein gaseous acetylene is burned at constant volume in the presence of a gaseous oxide of nitrogen within a combustion chamber having an outlet containing a seal, such as a diaphragm, which confines the combustion gas within the chamber until the chamber pressure rises to a pre-determined level and then abruptly releases the high temperature gas for supersonic flow through a nozzle and over a test specimen situated within the nozzle passage.

This invention relates to a supersonic wind tunnel and method of operating same and more particularly to a supersonic wind tunnel which utilizes the combustion of a hydrocarbon fuel with one or more oxides of nitrogen in a constant volume combustion chamber.

It has been proposed in US. Patent 3,191,435 to utilize the reaction of nitrous oxide with carbon to produce gaseous products for aerodynamic testing. Since the combustion of carbon does not result in Water vapor, no water condensation shock can result in the expansion nozzle. However, the CO component results in molecular weight of test gas which is considerably different from air. In the present invention, the combustion and chemical decomposition produces some Water vapor so that the molecular weight of the test gas is close or substantially identical to air and the temperature in the nozzle is maintained high enough to prevent a condensation shock. In other words, by controlling the area ratio of the nozzle, the static temperature at the exit is maintained above that at which condensation shock will occur. The present invention also has the advantage that it is easier to mix gases in a constant volume expansion chamber than it is to uniformly burn carbon in such a chamber.

In particular, the invention relates to the reaction between acetylene and nitrous oxide for producing simulated flight conditions in the lower hypersonic region and the reaction between acetylene and a mixture of nitrous oxide and nitric oxide for the higher hypersonic region. The reaction between acetylene (C H and nitrous oxide (N 0) produces -a gas whose composition and properties (molecular weight and specific heat ratio) are very similar to those of air. If necessary, the products can be diluted with N to obtain this similarity. While other hydrocarbon may produce even closer similarity to air, the greater amount of heat released by the C H reaction is great enough to provide the energy required 'for true simulation of test conditions at hypersonic velocities. When the reaction takes place in a blow down type tunnel of high combustion chamber volume-to-throat area ratio, sufficiently slow pressure decay rates can be obtained to closely simulate steady state testing. By adding fuel to the oxides of nitrogen, the heat of combustion is added to the heat liberated by chemical decomposition and by using hydrocarbon fuels, the test gases can be made to closely simulate air.

It is therefore an object of the present invention to provide a supersonic wind tunnel and method of operation in which a hydrocarbon fuel is combusted with one sonic wind tunnel and method of operation in whichacetylene is combusted with one or more oxides of nitrogen, including N 0 and NO, to produce a test gas for the tunnel.

A further object of the invention is to provide a supersonic wind tunnel and method of operation in which nitrogen gas is added to the products of combustion of a hydrocarbon fuel with one or more oxides of nitrogen to produce a test gas very similar to air.

These and other objects of the invention not specifically set forth above will become readily apparent from the accompanying description and drawing in which:

FIGURE 1 is a sectional view of a supersonic wind tunnel showing the manner in which gases are supplied to the combustion chamber; and

FIGURE 2 is a graphic illustration of various test gas compositions as a function of simulated flight velocity.

Referring to FIGURE 1, the combustion chamber casing 9 of the supersonic tunnel 10 comprises sections 11 and 12 which are secured together by threads 13. A flow nOzZle 15 has its entrance end 16 threaded into an opening 17 in casing section 12 so that the throat section 18 of the nozzle can communicate with combustion chamber 19 within casing 9. The nozzle 15 has a divergent-test section 22 and a testing model 23 can be located at the exhaust end 24 of the divergent section. The dimension of the nozzle governs the speed of gas flow through the exhaust end of the test section, and the ratio between the throat section and the exhaust end is low enough to prevent condensation shock,

The combustion chamber 19 is cylindrical in cross section and has an electrode supported within an insulated sleeve 33 located in an opening in casing section 11. One end of an ignition wire 31 is connected by a screw 32 to the end of electrode 30 and the other end of wire 31 is secured to the casing section 12 by'screw fastener 34. A voltage source 39, such as a battery, is connected across storage condenser 40 and the condenser is charged by closing switch 41. One side of the condenser is connected by line 44 to electrode 30 and the other side is connected to the casing section 12 by line 45. The closing of switch 46 in line 44 causes the condenser to discharge through wire 31 and. thereby heat the wire to a temperature, such as l200-1500 R, which will initiate combustion in chamber 19.

A mylar diaphragm 48 is located across the entrance end of the nozzle in order to confine the gas within chamber 19 until a predetermined pressure, such as 1000 p.s.i., is reached in the chamber. The thickness and material of the diaphragm is such that the diaphragm will rupture and release the gas from the chamber at the predetermined pressure and the diaphragm can be replaced after each test. A pressure gauge 50 is connected with chamber 19 by passage 51 which contains a valve 52 and the pressure of the gases charged into the chamber before combustion can be determined from the gauge. Also, a vacuum pump 55 is connected to chamber 19 by passage 56 which contains valve 57 so that'the chamber can be evacuated before any gases are admitted to the chamber.

A gas inlet passage 60 open into chamber 19 and contains a valve 61 which is closed when the chamber is being evacuated by pump 55. A header space 62 separate ly connects four passage 65, 66, 67 and 68 to the passages 60 and these passages contain separate valves 70, 71, 72 and 73, respectively. Passage connects with tank 75 which contains a source of acetylene. conventionally, the acetylene (C H would be dissolved in acetone in tank 75 and is released from the acetone, usually at about 40'0 p,s.i when valve 70 is opened. Passage 66 connects with tank 76 which contains nitrous oxide (N in liquid form and the liquid boils off when valve 71 is opened to provide a supply of N 0 gas usually at about 800 p.s.i. Tank 77 contains nitric oxide (N0) gas, which is usually stored at about 2200 p.s.i., and tank 78 contains nitrogen gas usually stored at about the same pressure. When the main valve 61 is open, any one of the tanks 75-78 can be connected to the chamber 19 by opening the valve in the separate line leading from each tank to the common header space 62 and the amount of each gas introduced into chamber 19 can be controlled by the increased pressure as read on gauge 50 as each gas is added to the chamber.

If only N 0 were added from tank 76 and decomposed in chamber 19 at constant volume into N and 0 the resulting gas would contain 36.4% oxygen and 63.6% nitrogen and additional inert nitrogen N could be added from tank 78 to adjust the mixture to 21% oxygen so that this mixture would correspond to that of atmospheric air. The resulting test gas would achieve a maximum temperature of about 3300 R. which simulates a flight velocity of about 6200 f.p.s. For higher temperatures than can be attained directly from the oxides of nitrogen while still retaining 21% oxygen, a fuel is added to consume a part of the oxygen and to release heat, and such fuels are preferably selected from the hydrocarbons which contain carbon, hydrogen and nitrogen. Of the hydrocarbon fuels, acetylene C H consumes less oxygen in burning than does ethylene (C H or ethane (C H and yet all these fuels have generally the same heating value. Thus, higher temperatures can be attained by employing acetylene instead of the more nearly saturated or fully saturated homologs before reducing the final oxygen concentration to the desired limit. Acetylene is useful only in the gaseous phase because of its detonable characteristics as a liquid.

FIGURE 2 shows the mole fraction of constitutents in the product gas as a function of stagnation temperature and simulated flight velocity when acetylene is reacted with oxide of nitrogen (N0 or NO or both) and if required, diluted with N At the condition designated by line 100, pure N 0 is decomposed and diluted with N to obtain 21% O and 79% N as indicated by curves 101 and 102, respectively, and the resulting stagnation temperature is shown as 3300 R. In moving from the condition at line 100 to the condition at line 103, increasing amounts of N are replaced with C H so that the excess 0 is combusted to result in higher temperatures. The resulting gas mixture contains carbon dioxide, water vapor, oxygen and nitrogen as indicated below:

The amounts of CO and H 0 in the product gas are shown by curves 104 and 105, respectively, and the 0 percentage remains at 21% as indicated by curve 101. Mixtures of C H ,N O and N can be varied in this range to obtain velocities from about 6000 to 10,000 f.p.s. At the condition of line 103, all N has been eliminated.

In the range beyond line 103, increasing amounts of N 0 are replaced with NO and velocities above 10,000 f. p.s. can be obtained by substituting NO for some of the N 0. When pure NO and C l-I are utilized, velocities on the order of 13,000 f.p.s. can be achieved. The gas properties of the mixtures remain very similar to air over a major portion of the total velocity range. For instance, as the velocity increases to 9500 f.p.s., the molecular weight of the mixture is 29.8 as compared to 29 for air, while the ratio of specific heats ('y) is about 1.38 as compared with the value of 1.4 for air. If desirable, this can be compensated for by the addition of a small amount of inert gas, such as neon, to increase 7 and reduce the molecular weight.

To illustrate the results obtained by the invention, C H and N 0 were introduced into a 0.25 cu. ft. com- 4 bustion chamber after the chamber was evacuated. The chamber was charged to 18 p.s.i.a. with C H and the N 0 was added until a total pressure of 268 p.s.i.a. was obtained so that partial pressures were 18 p.s.i.a. C H and 250 p.s.i.a. N 0. When the mixture was ignited electrically, a pressure of about 4360 p.s.i.a. and temperature of 6280 R. were obtained which correspond to the condition at line 103 in FIGURE 2. A pressure of 250 p.s.i.a. for the N 0 is well below its condensation point and the pressure of (3 H; was calculated as follows:

250N2O+PC2H2:2PCO2+PH2O+250N2+XO2 and X ZP WDW and 250 (total 0 atoms)=4P+P-|-2X Substituting for X in the second equation above, the pressure of C H is P=17.75 p.s.i.a.

The present invention provides a large scale, constant volume testing facility which is economical to construct and inexpensive to operate since chemical propellants are utilized for heating. Constant volume testing has two significant advantages over constant-pressure testing; (1) for the same propellants, higher temperatures can be generated, and (2) extremely high pressures can be self-generating. Constant-volume testing is possible since all the reactants are initially in the vapor phase as introduced to chamber 19 and the initial charging pressure of each reactant does not exceed its saturation pressure. For the gases charged into the chamber, enough fuel is added to combust the excess oxygen so as to maintain 21% oxygen in the test product gases. The oxides of nitrogen (N 0 and NO) are preferably combusted with acetylene and in the case of N 0, decomposition and combustion will occur simultaneously. The particular oxides of nitrogen are chosen to obtain the desired temperature and velocity. While the resulting test gas product includes small amounts of CO and H 0, the CO is heavier and the H 0 is lighter in molecular weight than air so that they tend to balance out. It is understood that different hydro carbon fuel can be utilized without adding contaminates other than carbon and Water vapor to the resulting test mixture.

Various other modifications of the invention are contemplated by those skilled in the art without departing from the spirit and scope of the invention as hereinafter defined in the appended claims. 1

What is claimed is:

1. A supersonic wind tunnel comprising:

a combustion chamber;

a flow nozzle connected with said chamber;

means for blocking flow through said nozzle until a predetermined pressure develops in said chamber;-

supply passage means for introducing gas to said chamber;

a source of gaseous oxide of nitrogenconnected with said passage means for introduction into said chamber;

a source of gaseous acetylene fuel connected with said passage means for introduction into said chamber; and

ignition means for initiating combustion of said fuel and oxide in said chamber to product a-high temperature, high pressure gas mixture which flows through said nozzle after developing a pressure sufficient to remove said blocking means.

2. A supersonic wind tunnel as defined in claiml wherein said source of fuel comprises a tank for supplying gaseous acetylene under .pressure, and .means for communicating said tank to said passage means.

3. A supersonic wind tunnel as defined in claim I having gauge means for indicating the partial pressures of said oxide and said fuel admitted to said chamber to obtain the desired reaction within said chamber.

4. A supersonic Wind tunnel as defined in claim 1 wherein said oxide source comprises separate tanks for supplying, respectively, gaseous nitrous oxide and gaseous nitric oxide under pressure, and means for selectively communicating said tanks to said passage means.

5. A supersonic wind tunnel as defined in claim I having a source of pure nitrogen gas connected with said passage means.

6. A supersonic wind tunnel as defined in claim 1 having:

gauge means for indicating the partial pressures of said oxide and said fuel admitted to said chamber to ob tain a desirable reaction within said chamber; and

vacuum pump means connected with said combustion chamber for evacuating said chamber prior to charging said chamber with oxide and fuel.

7. A supersonic wind tunnel as defined in claim 6 wherein said source of fuel comprises a tank for supplying gaseous acetylene under pressure, and means for communicating said tank to said passage means; and said oxide source comprises separate tanks for supplying, respectively, gaseous nitrous oxide and gaseous nitric oxide under pressure, and means for selectively communicating said latter tanks to said passage means.

8. A supersonic wind tunnel as defined in claim 1 wherein said oxide source comprises a tank for supplying gaseous nitrous oxide under pressure, and means for communicating said tank to said passage means.

9. A method of operating a supersonic wind tunnel consisting of a flow nozzle and a combustion chamber connected with sources of oxide of nitrogen and acetylene fuel comprising the steps of:

introducing said fuel and oxide of nitrogen to said chamber;

confining within said chamber the mixture of said oxide and said fuel;

igniting the confined mixture of said oxide and said fuel to cause combustion of said mixture within said chamber, whereby said combustion occurs within a constant chamber volume; and

releasing the gaseous combustion products from said chamber to said nozzle as a predetermined chamber pressure.

10. A method as defined in claim 9 wherein the steps of introducing said oxide and said fuel comprise charging said chamber to a preselected partial pressure for each to provide a resulting gaseous product containing substantially 21% oxygen and having the desired product temperature.

11. A method as defined in claim 10 wherein said oxide comprises nitrous oxide.

12. A method as defined in claim 10 wherein the said oxide comprises a mixture of nitrous oxide and nitric oxide.

13. A method as defined in claim 11 including the step of charging said chamber with nitrogen gas to provide a resulting gaseous product having substantially 79% nitrogen and 21% oxygen.

14. A method as defined in claim9 including the step of evacuating said chamber prior to introducing said oxide and said fuel.

15. A method as defined in claim 9 including the step of introducing an inert gas to said chamber to adjust the combustion products so that the gas properties of the products more closely approach those of air.

References Cited UNITED STATES PATENTS 6/1965 Weller 73----147 7/1966 Johnson 73-12 DAVID SCHONBERG, Primary Examiner. 

